In the correct cell operation the perfect isolation of the zinc anode and the porous manganese dioxide cathode, both soaked by the electrolyte, has an outstanding role. This task is performed by the separator system which should ensure good ionic connection between the two electrodes.
The use of double layer separators in alkaline cells has been known for a long time. Jammett, in U.S. Pat. No. 3,463,669 applies semipermeable materials such as regenerated cellulose or starch to a felted nylon fabric. Carboxymethyl cellulose in the nylon mat serves as a setting agent. Water soluble polyvinyl alcohol fibers bond viscose rayon and non-soluble polyvinyl alcohol fibers into a thin mat which may contain nonionic surfactants as wetting agents.
Gilman discloses in U.S. Pat. No. 3,980,497 methods of sandwiching a cellophane membrane between two layers of absorbent material using carboxymethyl cellulose or the like as a bonding agent.
For rechargeable cells, Kordesch recommends in U.S. Pat. No. 4,091,178 a cellophane/Pellon laminate with the cellophane placed against the anode and the Pellon placed against the cathode.
The use of a fibrous separator material is required in several types of alkaline cells to maintain the required electrolyte concentration and to prevent portions of the cell from drying out during use.
While much research work has been done concerning the choice of an optimum separator system, the problems connected with the sealing of the bottom edge of the separator have not been solved so far to a satisfactory extent.
In cylindrical cells, in which the electrodes are arranged coaxially around each other, the separator takes a thin cylindrical form. There are known solutions for forming the separator as an upright cylinder with a closed bottom. Since separator materials are made generally of a multi layer structure of different materials, the required folding of the bottom end of the cylinder is associated with wrinkling of the materials and special measures have to be taken to overcome such difficulties.
In Swiss patent No. 669,479 issued to Ruetschi, a multi layer separator cylinder was used, in which a hot melt sealant served to close the bottom. In a first embodiment, the sealant was applied on the top of the folded bottom portion; in a second embodiment the lower edges of the cylindrical separator were folded back and a plastic disc with stepped diameter was attached from above to the folded lower rim of the separator and the hot melt was applied between the stepped portion of the disc and the interior of the separator cylinder.
In all embodiments of the Ruetschi patent, the separator cylinder was mechanically reinforced by two perforated metal cylinders between which it was sandwiched. In the separator system itself a fibrous cylinder was used which formed the outer one of the multi layer system, i.e. it faced the cathode.
In the third embodiment, a disc was placed to fit in the bottom portion of the outer metal cylinder and the hot melt was applied on the top of the disc and filled the interior of the sandwiched structure up to a predetermined height.
The so-obtained reinforced and closed cylindrical separator system was thereafter inserted in the cathode cylinder as a ready made unit, i.e. in the Ruetschi patent the old principle of providing a separator cylinder closed at the bottom was utilized.
There are requirements concerning the separator system which cannot be harmonized with the above principle; e.g. it is often beneficial if the layers are wound to form the separator, but the mechanical strength of such a structure is insufficient for supporting a closed bottom as a self contained unit. The use of perforated metal cylinders at either side of the separator has been overcome by technical progress, and the lack of such a reinforcement system imposes further difficulties in following the above disclosed principles.
A further problem connected with the use of a separator system closed at the bottom lies in the inefficient utilization of available active cathode and anode volumes. The cathode cylinders can extend to the bottom of the cylindrical can of the cell, however, a think plastic disc is often used as an abutment member at the lower end of the cathode. The separator cylinder which is closed at the bottom and made as a self contained unit can be inserted in the hollow interior of the cathode up to a depth where the bottom face of the cylinder abuts the upper face of the plastic disc--i.e., the bottom edge of the cathode cylinder is at the same level as the bottom edge of the separator system. In such a separator system, the bottom sealing or closure require a certain height, which is not negligible compared to the full height of the cell. A certain height is required for the folding back of the bottom edge of the separator, for receiving the sealant which has to be self-supporting. This height zone can be considered as a volume loss, since the cathode material encircling it cannot actively contribute to the cell function.
The object of the present invention is to provide a cylindrical manganese dioxide-zinc cell with improved bottom sealing, wherein the requirements imposed on the separator system can be fulfilled without regard to the manner in which the bottom sealing is made, and which allows a better utilization of available cell volume.
It has been found according to the present invention that most of aforementioned problems can be overcome, if the separator system is inserted as a hollow upright cylinder open at both ends into the hollow inner space of the cathode to extend up to the end of the cathode, and a thermoplastic sealant is applied at this phase of the cell assembly to cover the bottom edge zone of the separator system and to impregnate it in this zone.
The advantages of this kind of bottom sealing are clearly understandable, since there will be no need for folding back the bottom end of the separator and there will be no need for making the separator as a self contained unit. A further advantage lies in that the sealant does not take a significant volume, and for that reason the whole cathode and anode mass can participate in the cell operation.
It is preferable that a disc or ring-like insulator washer be placed between the bottom of the can and that of the cathode, and that the separator system and the sealant extends at the contact zone of the edge of the separator and the washer and takes the form of a thin concave bead. In the ring-like design, the sealant can cover the inner surface of the tip of the can below the washer.
In the case of low mercury or mercury free cells, it is preferable that the separator system comprises a wound structure of a first layer of a fibrous sheet material that can be soaked by the electrolyte and a second layer having a microporous structure and being impervious to solid particles that can be present in the cell. Both layers are ion permeable, and the first layer contacts the anode.
In that arrangement, it is preferred if the second layer encircles the first one so that all paths of solid particles between spaces separated by the separator, including those that lead through edge regions of the fibrous first layer, are blocked by the second layer.
The thermoplastic sealant can be asphalt or any kind of commercially available hot melt.
The basic advantages of a cell made according to the invention will be apparent if the cell is a secondary one. However, the invention can well be used in case of primary cells as well.